In my prior applications 07/474,285 filed Feb. 2, 1990 (now abandoned) and 07/549,351 filed Jul. 6, 1990 (now U.S. Pat. No. 5,027,632), there is disclosed the production of small size steel reinforcing bars by a slit rolling method in which a substantial increase in speed is obtained by eliminating twisting of the advancing rod prior to its introduction to the slitter station. Such a method is referred to as the no twist slit-rolling approach method or NTA method.
In the production of small size steel reinforcing bars using the slit rolling method, a substantial increase in rolling speeds can be achieved by the NTA method, as previously noted, and in order to reduce unnecessary down time, there now exists a substantial need to successfully and reliably achieve the increased rolling speed of the NTA method.
In slit rolling, the advancing bar supplied to the slitter station has a "clover" cross section. In the slitter station, the "clover" section of the bar is rolled in a rolling stand to a "peanut" section and the bar of "peanut" section is supplied to a slitter stand where the bar is longitudinally divided into two identical sections.
In a single strand rolling operation (without slitting), in order to eliminate tension in the bar, loops are purposely formed in the steel bar between successive stands. An optical sensor is used to detect the size of the loop and an output signal of the sensor is used to regulate the speeds of the mill motors in the successive stands (generally the downstream strand) to maintain a stable loop.
In slit rolling, two parallel sections emerge from the same stand after slitting. When the two loops of the slit sections are not the same, two different signals, would be produced and the control equipment would not be able to function.
While off line visual checks of alignment can give a good approximation, a small deviation of the alignment of guide means for the bar during rolling will give rise to unequal slitting. This will result in two problems.
1) The two finished bars will be different in weight. This produces a less desirable product as market requirements are for a 1/2 DIN tolerance.
2) In extreme cases, the loop growth of one of the slit sections may become uncontrollable.
In both cases, stoppage of production is inevitable.
At present, the highest rolling speed claimed for delivery of a single strand onto a cooling bed is about 20 M/s. This corresponds to a slitting operation at about 6.4 M/s. For slit rolling of 10 mm bars, the section being slit is a "clover" with a side dimension of 25 mm. A lateral deviation of 0.1 mm of alignment of the entry guide to the slitter station would produce an imbalance of 1.6% in the two slit sections or strands of the bar. At 6.4 M/s rolling speeds, this would mean the loop of the larger slit section will increase 102 mm per second more than the smaller slit section. For a typical 1,000 kg billet, the rolling time through the finishing stand, after slitting is 48.1 seconds. This means that before the strands are through the last finishing stands, the difference in length of the two loops would be about 4.3 meters. Because of such difference in length, the parallel loops cannot be compensated by varying the speeds of the mill motors, without causing undesirable tension on the smaller slit section.
The above evaluation is only to illustrate the sensitivity of the effect of imbalanced strands. In practice, the imbalance would be substantially higher than the example given. A system of NKK of Japan is known for adjusting a guide by manual means from a control pulpit. While this may be acceptable in low speed slit rolling, human response would not be fast enough for high speed rolling.